Helium

Thanks avinashtechlvr for the PR!

Helium ships with its own copies of gecko- and chromedriver. These binaries quickly become outdated. Previously, there was no easy way to "fix" Helium's webdrivers for your machine.

No longer. Now, Helium first attempts to use `chromedriver` and `geckodriver` from your `PATH`. This lets you install the versions of those drivers that work on your system.

The implementation is based on a PR by TotallyNotChase. Thank you!

See the new test [`test_drag_point`](https://github.com/mherrmann/selenium-python-helium/blob/ffdac9349ba52d74ec45da1cc12880a5170dcbaf/tests/api/test_point.py#L95-L99).

Thank you ftnext for the PR!

Added an argument `options` to `start_chrome(...)`. You can use it to supply the `ChromeOptions` when starting the browser. For example:

python
from selenium.webdriver import ChromeOptions
options = ChromeOptions()
options.add_argument('--start-maximized')
options.add_argument('--proxy-server=1.2.3.4:5678')
start_chrome(options=options)

Thank you Tim Powell (tbp105) for the PR (2)!